1. Field of the Disclosure
The present disclosure relates generally to catalytic converters, and more particularly to a diesel oxidation catalyst for decreasing sulfur trioxide emissions and consequently sulfuric acid mist.
2. Background Information
One problem faced in the treatment of diesel engine exhaust is presented by the presence of sulfur in diesel fuel. Upon combustion, sulfur forms sulfur dioxide (SO2), which a diesel oxidation catalyst (DOC) catalyzes to sulfur trioxide (SO3) with subsequent formation of condensable acidic sulfur compounds, such as sulfuric acid, which condense upon and thereby add to the mass of particulates. Additionally, sulfuric acid can cause corrosion of the downstream exhaust system leading to perforation and damage to components. DOCs are also generally used for burning (oxidizing) HC and CO as well as some of the soluble organic compounds that are adsorbed on soot particles. Additionally, DOCs may be used for oxidizing nitrous oxides (N2O) to nitrogen dioxide (NO2). Under certain conditions, presence of NO2 may not be favorable, such as in mining or other underground conditions, because the NO2 may be toxic. On the other hand, in other conditions, especially when a DPF is located after the DOC, NO2 formation may be favorable because it may oxidize soot which may help to regenerate the DPF.
Typical DOCs include a coating of one or more carrier material oxides, such as aluminum oxide, silicon dioxide, titanium dioxide, and cerium oxide, as well as mixtures thereof; one or more zeolites for absorbing HCs; and platinum, in addition to small amounts of palladium, in ratios of about 15:1, 10:1, 5:1, 3:1, 2.5:1, and 1:1, employed as a catalytically active component on a ceramic or metal substrate. The employed carrier material oxides have a large surface area of about 150 m2/g to about 300 m2/g and may generally remain stable up to exhaust gas temperatures of about 800° C. In order to achieve a high degree of catalytic activity, the catalysts are distributed very finely on the carrier material oxides, producing high amounts of SO3. Deterioration in activity of the oxidation catalyst caused by sulfur from SO3, as well as contamination and corrosion from sulfuric acid mist, are a major concern.